1. Field of the Invention
The present invention concerns a method for the control of the pixels of a plasma panel by means of semi-selective and selective addressing phases. The invention can be applied to AC type panels with coplanar sustaining, particularly of the type wherein each elementary picture element is defined substantially at the intersection of an addressing electrode, called a column electrode, with two other parallel electrodes forming a pair of sustaining electrodes.
2. Description of the Prior Art
Plasma panels are flat panel or screen display devices that enable the display of alphanumerical, graphic or other images, in color or otherwise. These panels work on the principle of an emission of light produced by an electrical discharge in a gas.
Generally, plasma panels comprise two insulating plates bounding a volume occupied by a gas (generally a neon-based mixture). These plates support conductive electrodes intersecting so as to form a matrix of cells each forming a picture element or pixel. An electrical discharge in the gas, causing an emission of light at a cell or pixel, takes place when the electrodes of this pixel are suitably excited.
Although certain plasma panels work in DC mode, it is most commonly preferred to use AC type panels, the working of which is based on an excitation of the electrodes in AC mode. The electrodes are coated with a layer of dielectric material They are therefore no longer in direct contact either with the gas or with the discharge.
The working of an AC type plasma panel with two intersecting electrodes to define a pixel is known, notably from a French patent No. 7,804,893, filed on behalf of THOMSON-CSF and published under No. 2,417,848.
With a view, notably, to improving the luminance of the plasma panels and also to enabling the display of several colors, it is preferred to use plasma panels of the type excited in AC mode as mentioned above but which, in addition, have coplanar sustaining.
In panels of this latter type, called coplanar sustaining plasma panels, each pixel of the matrix is formed by three electrodes, more precisely at the intersection between an addressing electrode, called a column electrode, and two parallel sustaining electrodes forming a pair of sustaining electrodes. With this type of panel, it is known that the sustaining of the discharges is done between the two sustaining electrodes of one and the same pair, and that the addressing is done by the generation of discharges between two intersecting electrodes.
The sustaining electrodes form two classes: the electrodes of a first class are called "addressing-sustaining" electrodes, while the electrodes of a second class are called "solely sustaining electrodes". The function of the addressing-sustaining electrodes is, firstly, in cooperation with the solely sustaining electrodes, to achieve the sustaining discharges and, secondly, to fulfill an addressing role. Consequently, they are individualized, that is, they are connected to one or more pulse generating devices through means that enable one or more particular pulses, called addressing pulses, to be applied to only one or to more addressing-sustaining electrodes which are selected from among the plurality of addressing-sustaining electrodes.
The solely sustaining electrodes (of the second class) are generally connected to one or more pulse generators in such a way that these solely sustaining electrodes are all, at the same instants, carried to the same potentials, so that they do not need to be individualized and may, if necessary, be connected to one another.
The term "addressing" refers to the signals applied to the electrodes of one or more pixels selected from among the plurality of pixels, in order to obtain their writing (lighting up) and/or their erasure (extinguishing). This is by contrast with the sustaining signals which are applied, without distinction, to the electrodes of all the pairs of sustaining electrodes in order to provoke sustaining discharges (light emission) by all the pixels which are in the written state.
The addressing may be selective or semi-selective:
the addressing is selective when it determines either the writing or the erasure of one or more selected pixels, without modifying the state of the other pixels belonging to a same line-up as the pixel or pixels selected (a line-up of pixels may be formed either in the direction of a row of pixels, that is, parallel to the pairs of electrodes, or in a direction perpendicular to the rows, that is, parallel to the column electrodes. PA1 the addressing is semi-selective when it either writes or erases, simultaneously, an entire line or line-up of pixels (the line-up may be parallel to the pairs of electrodes or parallel to the column electrodes). It must be noted that should a control method include a semi-selective addressing phase (either for a writing operation or for an erasing operation) this first semi-selective addressing phase is generally followed by a selective addressing phase (which achieves the opposite operation). PA1 a semi-selective addressing phase during which an entire row of pixels is erased; PA1 the semi-selective addressing phase is followed by a stabilization phase (optional); PA1 then, a selective addressing phase, during which only the selected pixel or pixels are written; PA1 then, a specific sustaining phase.
Among the advantages provided by the structures where the pixel is defined at the intersection of a column electrode with a pair of sustaining electrodes, we might cite greater luminance. This is due notably to the fact that the sustaining discharges (which are those giving the essential part of the light) between the two sustaining electrodes, occur on a surface that goes beyond the surface of intersection with the column electrode. This means that the useful light is not blocked by this column electrode which is generally mounted on the side with the plate by which the plasma panel is looked at.
It must be noted that the addressing/sustaining electrodes and solely sustaining electrodes each have, at each pixel, a protuberance or projecting surface. In one and the same pair of sustaining electrodes, the projecting surfaces of one electrode are pointed towards the projecting surfaces of the other electrode, and the sustaining discharges occur between these projecting surfaces.
A plasma panel such as this is known notably from the European patent document EP-A-O 135 382 which also describes a method for the control of this panel. It must be noted that, in the device described in this European patent, the column electrode intersects the pairs of sustaining electrodes on the side of the projecting surfaces where the sustaining discharges are produced.
Another structure of the type wherein each pixel is defined at the intersection of a column electrode with a pair of sustaining electrodes, as well as an adapted control method, are described in the article by G. W. DICK in PROCEEDINGS OF THE SID, vol 27/3, 1986, pages 183-187. It must be noted that, in the structure described in this document, the sustaining electrodes have a constant width, that is, they have no facing, projecting surfaces in a pair of sustaining electrodes, to define the sustaining discharge zone. By contrast, this structure has barriers made of an insulating material. These barriers serve to confine sustaining discharges in the zone of intersection with the column electrode.
Another type of plasma panel, to which the method of the invention can be applied in a particularly worthwhile way, is shown in FIG. 1. A panel of this type is the object, in itself, of a French patent application No. 88 03953 filed on Mar. 25, 1988 on behalf of THOMSON-CSF. Since this French patent application has not been published to date, the new type of plasma panel to which it refers is described hereinafter.
The panel shown in FIG. 1 has a first glass plate 10 covered with a first class of electrodes marked Xj where j is a whole number ranging from 1 to N (only one electrode Xj is shown; the set formed by the plate 10 and the electrode Xj is coated with a layer 12 of dielectric material, which may be covered with a layer of oxide such as MgO (not shown). On the dielectric layer 12, there is a patch 14 of a luminophor material, namely a material capable of emitting a colored radiation under the effect of an ultra-violet radiation.
The panel further has a second glass plate 20 coated with a second class of electrodes formed by pairs of electrodes, respectively called sustaining-addressing electrodes (Yae)i and sustaining electrodes (Ye) where i is a whole number in the range of 1 to P. The sustaining-addressing and sustaining electrodes include protuberances or projecting surfaces 22 and 24, placed so as to face each other. The unit formed by the plate 20 and the electrodes is coated with a dielectric layer 26.
In normal operation, the two plates 10 and 20 and their networks of electrodes are brought close together and kept apart by a shim (not shown), and there is a gas present in the volume between the plates and the shim. Once the panel is mounted, it thus has two networks of orthogonal electrodes, in the sense that the electrodes Xj are orthogonal to the electrodes (Yae)i and (Ye). The electrodes Xj may overlap the protuberances 22 and 24, or may be slightly offset on their side. A pixel Pij is then defined by an electrode Xj (a column electrode) and a pair of sustaining electrodes (Yae)i and (Ye).
If the above-described plasma panel or other coplanar sustaining AC type plasma panels, such as, for example, the panels described earlier, are controlled by a known control method, it is observed, notably, that the working of these panels is too limited as regards the speed with which an image can be renewed, to be capable of being used as a so-called "all options" display panel, namely to display an image with a sufficient number of half-tones or gradual ranges of colors. For, especially with the making of color panels, it becomes very important to have a large number of half-tones (128 for example) to make a good picture (of the television cathode-ray tube picture type) on a plasma panel with a number of rows of pixels at least equal to 512.
The time needed to form a picture depends on the number of pixels and on the time needed for the erasure addressing, writing addressing and sustaining operations.
To reduce the time needed to form a picture, it is sought to reduce the total addressing time. To this effect, the prior art method consists in doing a semi-selective addressing (either for erasure or for writing, and in rows or columns) followed by a selective addressing.
Thus, for example, assuming that the semi-selective addressing concerns the erasing operation and is done according to rows of pixels, a basic cycle or cycle period per line generally comprises:
To each of these phases, there corresponds a particular combination of voltages developed among the three electrodes that form a pixel, following the application to one or more of these electrodes of positive or negative pulses forming sets of cyclical pulses.
This is repeated for each line of pixels.
It would seem that, at present, the minimum period that may be expected for a basic cycle as defined above is of the order of 20 microseconds.
Thus, in the case, for example, of a plasma panel with 512.times.512 pixels, if the image is renewed at 50HZ, only four half-shades are possible, in view of the method used for the control of the half-shades.